Single-substrate-processing CVD apparatus and method

ABSTRACT

A single-substrate-processing CVD apparatus is used for forming a BST thin film on a semiconductor wafer while supplying a first process gas containing a mixture of Ba(thd) 2  and Sr(thd) 2 , and a second process gas containing Ti(O-iPr)(thd) 2  or Ti(thd) 2 . Precursors of Ba and Sr have lower activation energies and higher resistivities than precursors of Ti. The first and second process gases are supplied from a shower head which has a group of first spouting holes for spouting the first process gas and a group of second spouting holes for spouting the second process gas. The group of the second spouting holes are designed to have diameters gradually decreasing in radial directions outward from the center of a shower region, such that the second process gas is supplied at a spouting rate gradually decreasing in radial directions outward from the center.

BACKGROUND OF THE INVENTION

The present invention relates to a single-substrate-processing CVD(Chemical Vapor Deposition) apparatus and method, and particularly to anapparatus and method for growing an insulating, high-dielectric, orferroelectric film by means of MOCVD (Metal Organic Chemical VaporDeposition).

In order to manufacture semiconductor devices, film formation andpattern etching are repeatedly applied to a semiconductor wafer. Assemiconductor devices are increasingly highly miniaturized andintegrated, demands on the film formation become more strict. Forexample, very thin insulating films, such as capacitor insulating filmsand gate insulating films are still required to be thinner and to bemore insulating.

Conventionally, silicon oxide films and silicon nitride films are usedas the insulating films. In recent years, however, it has been proposedto form the insulating films from materials having more excellentinsulating properties, such as metal oxides, e.g., tantalum oxide (Ta₂O₅), or high-dielectric or ferroelectric bodies containing two metalelements or more, e.g., (Ba,Sr)TiO₃, i.e., BST. These films can beformed by means of MOCVD, i.e., using vaporized metal organic compounds.

When a film is formed by means of CVD, it is important to maintain ahigh planar uniformity in the film thickness, in light of attaining ahigh yield and the like. For this reason, process gases are suppliedfrom gas spouting holes arranged at uniformly distributed positions on ashower head, so that the process gases are uniformly supplied onto thesurface of a wafer. Where a CVD process is performed to form a film of atwo-element material including only one metal or semiconductor element,represented by SiO₂ or TiN, it is possible to maintain a high planaruniformity in the film thickness by such a shower head.

According to experiments conducted by the present inventors, however,where a CVD process is performed to form a film of a composite materialincluding two metal elements or more, such as a BST thin film, planeruniformity in the film composition is sometimes lowered by thisconventional CVD process. Like planar uniformity in the film thickness,planar uniformity in the film composition, i.e., the ratio of metalelements, is also an important issue, in light of attaining a high yieldand maintaining electric properties.

Further, where a tantalum oxide film is formed by means of MOCVD, a rawmaterial gas containing tantalum alkoxide, and oxygen gas are used. Inthis case, reaction byproducts, such as organic substances, e.g., CH₃CHO, are slightly mixed into the tantalum oxide film, therebydeteriorating the film's properties, such as the insulation breakdownvoltage.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide asingle-substrate-processing CVD apparatus and method which can improveplanar uniformity in film composition, where a film of a compositematerial including two metal elements or more is formed.

Another object of the present invention is to provide asingle-substrate-processing CVD apparatus and method which can reducethe amount of reaction byproducts contained in a film to be formed bymeans of CVD.

According to a first aspect of the present invention, there is provideda single-substrate-processing CVD apparatus for forming a thin filmcontaining first and second metal elements on a target substrate whilesupplying first and second process gases containing the first and secondmetal elements, respectively, a precursor of the first metal elementproduced by dissociation of the first process gas having a loweractivation energy and a higher resistivity than a precursor of thesecond metal element produced by dissociation of the second process gas,the apparatus comprising:

an airtight process chamber;

a worktable arranged in the process chamber to mount the targetsubstrate thereon;

an exhaust system configured to exhaust the process chamber; and

a supply system having a shower head configured to supply the first andsecond process gases into the process chamber, wherein the shower headhas a shower region facing the worktable, on which a group of firstspouting holes for spouting the first process gas and a group of secondspouting holes for spouting the second process gas are arranged, andwherein the supply system is designed such that the first process gas issupplied at a spouting rate substantially uniform over the showerregion, and the second process gas is supplied at a spouting rategradually decreasing in radial directions outward from a center of theshower region.

According to a second aspect of the present invention, there is provideda single-substrate-processing CVD method of forming a thin filmcontaining first and second metal elements on a target substrate whilesupplying first and second process gases containing the first and secondmetal elements, respectively, a precursor of the first metal elementproduced by dissociation of the first process gas having a loweractivation energy and a higher resistivity than a precursor of thesecond metal element produced by dissociation of the second process gas,the method comprising:

placing the target substrate on a worktable arranged in an airtightprocess chamber; and

forming the thin film on the target substrate by supplying the first andsecond process gases into the process chamber from a shower regionfacing the worktable while exhausting the process chamber, wherein thefirst process gas is supplied at a spouting rate substantially uniformover the shower region, and the second process gas is supplied at aspouting rate gradually decreasing in radial directions outward from acenter of the shower region.

According to a third aspect of the present invention, there is provideda single-substrate-processing CVD apparatus for forming a thin filmcontaining a metal element on a target substrate while supplying firstand second process gases containing the metal element and a non-metalelement for combining with the metal element, respectively, theapparatus comprising:

an airtight process chamber;

a worktable arranged in the process chamber to mount the targetsubstrate thereon;

an exhaust system configured to exhaust the process chamber; and

a supply system having a shower head configured to supply the first andsecond process gases into the process chamber, wherein the shower headhas a shower region facing the worktable, on which a group of firstspouting holes for spouting the first process gas and a group of secondspouting holes for spouting the second process gas are arranged, andwherein the group of the first spouting holes are arranged to distributein a first zone having a surface area substantially smaller than asurface area of the target substrate facing the shower region, and thegroup of the second spouting holes are arranged to distribute in asecond zone concentric with the first zone and having a surface areasubstantially the same as or larger than the surface area of the targetsubstrate.

According to a fourth aspect of the present invention, there is provideda single-substrate-processing CVD method of forming a thin filmcontaining a metal element on a target substrate while supplying firstand second process gases containing the metal element and a non-metalelement for combining with the metal element, respectively, the methodcomprising:

placing the target substrate on a worktable arranged in an airtightprocess chamber; and

forming the thin film on the target substrate by supplying the first andsecond process gases into the process chamber from a shower regionfacing the worktable while exhausting the process chamber, wherein thefirst process gas is supplied from a first zone having a surface areasubstantially smaller than a surface area of the target substrate facingthe shower region such that the first process gas is supplied at aspouting speed higher than a spouting speed at which the first processgas would be supplied from a zone having the same surface area as thesurface area of the target substrate when an equal supply rate is used,and the second process gas is supplied from a second zone concentricwith the first zone and having a surface area substantially the same asor larger than the surface area of the target substrate.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1 is a diagram schematically showing a CVD apparatus according to afirst embodiment of the present invention;

FIG. 2 is a diagram showing the main part of the CVD apparatus shown inFIG. 1;

FIG. 3 is a view showing the shower region of a shower head employed inthe CVD apparatus shown in FIG. 2;

FIG. 4 is a view showing the flow and composition change of processgases spouted from the shower head of the CVD apparatus shown in FIG. 2;

FIG. 5 is a cross-sectional view schematically showing a shower headaccording to a modification;

FIG. 6 is a graph showing relationships between the position on a waferand Ti/(Ba+Sr) in BST thin films formed by a method according to thepresent invention and a conventional method;

FIG. 7 is a view showing the flow of all the gases in a process chamber;

FIG. 8 is a view showing the flow and composition change of processgases where a thin film of a composite material is formed by aconventional method;

FIG. 9 is a diagram showing a CVD apparatus according to a secondembodiment of the present invention;

FIG. 10 is a plan view showing the bottom surface of a shower head shownin FIG. 9;

FIG. 11 is a graph showing the result of evaluating the insulationbreakdown voltage of tantalum oxide films;

FIGS. 12A and 12B are views each showing the result of simulating theconcentration distribution of a raw material gas (Ta(OC₂ H₅)₅ : Pet);

FIGS. 13A and 13B are views each showing the result of simulating theconcentration distribution of a reaction byproduct (CH₃ CHO);

FIG. 14 is a graph showing the result of simulating the concentrationdistribution of the raw material gas;

FIG. 15 is a graph showing the result of simulating the concentrationdistribution of the reaction byproduct; and

FIG. 16 is a graph showing the result of simulating the deposition rateof tantalum oxide films.

DETAILED DESCRIPTION OF THE INVENTION

In the process of developing the present invention, the presentinventors conducted research on the reasons as to why a CVD filmcontaining a plurality of metal elements, such as a BST film, entailed alow planar uniformity in the film composition. As a result, the presentinventors have obtained the following findings.

Where a composite material thin film is formed by means of CVD, aplurality of process gases need to be supplied into a CVD processchamber and simultaneously cause a film deposition reaction on thesurface of a wafer. However, raw material gases do not necessarily havethe same or similar reactivity. Where raw material gases having higherand lower resistivities are supplied onto the surface of a wafer whilethey are mixed and spouted from spouting holes having a uniform diameterdistribution, the formed film has a low planar uniformity in the filmcomposition.

Especially, where a BST thin film is formed on a wafer, planaruniformity in the film composition becomes lower with a decrease in thefilm deposition temperature, such that the film is gradually Ti enrichedfrom the center to the edge of the wafer. In recent practical processes,film deposition tends to be performed under a lower temperature toattain an excellent step coverage. As a result, planar uniformity in thefilm composition tends to be lowered, and so does planar uniformity inthe film properties.

According to an analysis on the film deposition mechanism of BST-CVDconducted by the present inventors, it has been found that precursors ofTi have activation energies far higher than those of precursors of Baand Sr in the film deposition reaction. Accordingly, the growth rate ofTi is reduced with a decrease in temperature. On the other hand,(Ba,Sr)TiO₃ is formed of a solid solution of BaTiO₃ and SrTiO₃, and thusshould have a stoichiometric ratio of (Ba+Sr):Ti=1:1. In other words,where this substance is formed by means of CVD, it is necessary to causeTi to have a growth rate higher than each one of those of Ba and Sr, andequal to the sum of those of Ba and Sr. For this reason, a raw materialgas for Ti has to be supplied in large quantities.

Further, the present inventors have found the following phenomena bysimulating the flow and concentration distribution of gases between ashower head and the surface of a wafer, where a CVD process is performedto form a film of a two-element material including only one metal orsemiconductor element, represented by SiO₂ or TiN. Specifically, asshown in FIG. 7, where gas spouting holes arranged on a shower head 6have the same diameter, the flow rates of process gases from thespouting holes are equal to each other, when the process gases aresupplied into a process chamber 2. The process gases reach the surfaceof a wafer by flow and diffusion in a vertical direction from the showerhead 6 to the wafer surface, then perform a film deposition reaction onthe wafer surface. The inside atmosphere in the process chamber isexhausted from exhaust ports 8 arranged around a worktable, so that theprocess gases partly flows in lateral directions. As a result, the flowof all the gases in the process chamber 2 is formed as shown in FIG. 7.Flow rates of gases in lateral directions due to gas exhaust are smallerat the center of the shower head 6 than at positions near the edge.

Incidentally, in order to obtain a high deposition rate, it is necessaryto supply a great amount of the raw material gases into the processchamber to increase the gas concentration on the wafer surface. In thiscase, the process gases supplied on the wafer surface are not completelyconsumed, so non-reacted parts of the process gases flow outward inparallel to the wafer surface so as to join gases flowing downward fromthe shower head 6 and to perform a film deposition reaction. The gasessupplied from spouting holes on the periphery of the shower head 6 areimparted with high flow rates in lateral directions due to gas exhaust,thereby reducing the amount which reaches the periphery of the wafersurface. Seemingly, the growth rate of the film on the periphery of thewafer surface is smaller than that on the center of the wafer surface.But, that is not the case, because non-reacted parts of the processgases flowing from the center join the gases flowing from above, andcompensate for the gas concentration to maintain the growth rate of thefilm.

In the case of a film of a two-element material, such as SiO₂ or TiN, ahigh planar uniformity in the film thickness can be obtained, if auniform concentration of raw material gases and a uniform temperatureare held on the surface of a wafer. However, in the case of a film of acomposite material represented by BST, not only planar uniformity in thefilm thickness, but also planar uniformity in the film composition hasto be maintained high.

FIG. 8 is a view showing the flow and composition change of processgases where raw material gases for Ba, Sr, and Ti are mixed with eachother in advance, and supplied from spouting holes 28 on a shower head 6into a process chamber at a constant rate. As shown in FIG. 8, theprocess gases at the center of maintain their original composition.However, precursors of the metals have different activation energies inthe film deposition reaction, depending on the metals, and thus presentdifferent deposition rates and different reaction transforming ratios,thereby changing the composition of non-reacted parts of the processgases. On the way to the edge of the wafer along the wafer surface, thenon-reacted gases join the gases flowing downward from the shower head6, and change the composition of the gases flowing downward. As aresult, the film formed on the wafer comes to have different filmcompositions between the center and periphery of the wafer.

This problem will be explained in detail with reference to the numericalvalues shown in FIG. 8. In FIG. 8, the numerical values shown not inparentheses denote flow rate ratios of the process gases, while thenumerical values shown within parentheses denote composition ratios ofthe metal elements. Precursors of Ti have far higher activation energiesand lower reactivities, than precursors of Ba and Sr. Accordingly, theraw material gas for Ti is supplied in large amounts, so that Ti has agrowth rate two times larger than each one of those of Ba and Sr. With adecrease in temperature, the reaction rate of Ti film deposition islowered, and the excessive amount of the raw material gas for Ti becomeslarger.

For example, it is supposed that the process gases are supplied from theshower head 6 to the wafer at ratios in the flow rate and composition ofBa:Sr:Ti=10:10:50 (1:1:5). In light of mass transfer in lateraldirections by flow and diffusion, the process gases reach the center ofthe wafer surface at ratios in the rate and composition ofBa:Sr:Ti=5:5:25 (1:1:5). Namely, the gas concentration decreases, butthe composition is maintained. Further, it is supposed in this case thatmass transfer of the process gases is performed in a non-dissociatedstate of the raw material gases for Ba, Sr, and Ti, because thetemperature other than that on the wafer surface is low, anddissociation of the process gases can not be caused. It is also supposedthat the raw material gases for Ba, Sr, and Ti diffuse and flow inlateral directions at the same rate, because the raw material gases havealmost the same molecular weight and similar molecular structures, andthus have similar diffusion coefficients.

If the film formed at the center of the wafer has a ratio in thecomposition of Ba:Sr:Ti=1:1:2, and the process gases are consumed at aratio of Ba:Sr:Ti=2:2:4 in the film deposition reaction at the center ofthe wafer, the process gases are left non-reacted at ratios in the rateand composition of Ba:Sr:Ti=3:3:21 (1:1:7). The non-reacted processgases flow outward in parallel to the wafer surface.

The process gases spouted from the spouting holes 28 located between thecenter and periphery of the shower head 6 receive forces in lateraldirections stronger than those to the gases at the center of the showerhead 6. Accordingly, the process gases spouted from the spouting holesat the in-between positions reach the wafer surface at the samecomposition, but at a smaller rate, as compared to the center. If theprocess gases reach the wafer surface at ratios in the rate andcomposition of Ba:Sr:Ti=3:3:15 (1:1:5), they join the non-reactedprocess gases from the center to form ratios in the rate and compositionof Ba:Sr:Ti=6:6:36 (1:1:6). As a result, the in-between positions on thewafer surface are provided with a sufficient amount of gases with thesupplement from the center, but is provided with a gas compositiondifferent from the original. Since the provided gases thereon are richerin Ti than the original, the composition of a film to be formed thereonis also richer in Ti than that on the center. Such a phenomenon isrepeatedly caused from the center to the edge of the wafer surface, sothe composition of the formed film ends up in being richer in Ti at aposition closer to the periphery of the wafer. Further, as describedabove, with a decrease in temperature, the raw material gas for Ti needsto be supplied in large amounts, but the growth rate of Ti is slower, sothe excessive rate of Ti is increasingly greater.

Embodiments of the present invention that are made on the basis of thesefindings will be described hereinafter with reference to theaccompanying drawings. In the following description, the constituentelements having substantially the same function and arrangement aredenoted by the same reference numerals, and a repetitive descriptionwill be made only when necessary.

FIG. 1 is a diagram schematically showing a CVD apparatus according to afirst embodiment of the present invention.

As shown in FIG. 1, the CVD apparatus includes a process chamber 2formed of a cylinder of, e.g., aluminum. A worktable 4 is arranged inthe process chamber 2, for mounting a semiconductor wafer W as a targetobject. A shower head 32 is arranged on the ceiling of the processchamber 2, for supplying process gases into the process chamber 2.Exhaust ports 8 are formed in the bottom of the process chamber, forexhausting used gases. The exhaust ports 8 are connected to an exhaustline 14 which is provided with a trap 10 for removing byproducts fromthe exhausted gas, and a pressure adjusting valve 12 for adjusting thepressure in the process chamber 2. The process chamber 2 isvacuum-exhausted by a vacuum pump (not shown) through the exhaust line.

In this embodiment, a thin film of (Ba,Sr)TiO₃, i.e., a BST thin film,is formed as an example of composite material thin films. As rawmaterials for Ba, Sr, and Ti, the following materials dissolved in asolution of butyl acetic acid or THF=tetrahydrofuran are used.Specifically, used for Ba is Ba(thd)₂=bis(tetramethylheptanedionato)barium=Ba(C₁₁ H₁₉ O₂)₂. Used for Sr isSr(thd)₂ =bis(tetramethylheptane-dionato)strontium=Sr(C₁₁ H₁₉ O₂)₂. Usedfor Ti is Ti(O-iPr)₂ (thd)₂=bis(isopropoxy)bis(tetramethyl-heptanedionato)titanium=Ti(C₃ H₇ O)₂(C₁₁ H₁₉ O₂)₂, or TiO(thd)₂=bis(tetramethylheptanedionato)oxotitanium=TiO(C₁₁ H₁₉ O₂)₂. Precursorsof Ba and Sr generated by dissociation of the raw material gases haveactivation energies similar to each other between Ba and Sr, and lowerthan those of precursors of Ti, and have reactivities higher than thoseof precursors of Ti. For this reason, the raw material gases for Ba andSr are mixed with each other before being supplied, while the rawmaterial gas for Ti is independently supplied. Further, oxygen (O₂) gasand argon (Ar) gas are also supplied as an oxidizing gas and a carriergas, respectively.

Liquids of Ba(thd)₂, Sr(thd)₂, and Ti(O-ipr)₂ (thd)₂ or TiO(thd)₂, arestored in tanks 16, 18, and 20, respectively, and are sent by pressureof Ar gas. Note that Ti(O-ipr)₂ (thd)₂ is used in this embodiment.

The Ba tank 16 and the Sr tank 18 are connected to a liquid pump 22through passages 34 and 36, respectively. The raw material liquids fromthe tanks 16 and 18 are mixed with each other, and sent by pressure intoa gas passage 38 connected to the shower head 32. The gas passage 38 isprovided with a vaporizer 24, to which Ar gas is supplied as a carriergas, while its flow rate is controlled by a mass-flow controller 40. Themixed raw material liquids are vaporized by the vaporizer 24 to form afirst process gas. The gas passage 38 is surrounded by, e.g., a tapeheater 26 to heat the gas flowing through the passage 38 at theliquefying temperature or more, so that the vaporized raw materials areprevented from returning to a liquid state.

The Ti tank 20 is connected to another liquid pump 44 through a passage42. The raw material liquid from the Ti tank 20 is sent by pressure intoa gas passage 46 connected to the shower head 32. The gas passage 46 isprovided with a vaporizer 48, to which Ar gas is supplied as a carriergas, while its flow rate is controlled by a mass-flow controller 50. TheTi raw material liquid is vaporized by the vaporizer 48 to form a secondprocess gas. The Ti raw material gas is not mixed with the other processgas in the shower head 32, but is mixed when it is spouted into theprocess chamber (in a manner of so called post-mixing supply). The gaspassage 46 is also provided with, e.g., a tape heater 52, so that thevaporized raw material is prevented from returning to a liquid state.

The shower head 32 is also connected to a passage 58 for supplyingoxygen gas as an oxidizing gas, along with Ar gas. The flow rates of theoxygen gas and the Ar gas are controlled by mass-flow controllers 54 and56, respectively.

The passages 38 and 46 for the raw material gases are connected tobypass passages 60 and 62, respectively, which communicate with anexhaust passage 14, so that unnecessary gases are exhausted withoutpassing through the process chamber 2. The passages 38, 46, 60, and 62are provided with switching valves 64 to open and close the passages, ifnecessary.

Argon gas having a certain pressure is supplied to each of the rawmaterial tanks 16, 18, and 20 for storing the raw material liquids, sothat the liquids of Ba(thd)₂, Sr(thd)₂, and Ti(O-ipr)₂ (thd)₂ are sentby pressure into the passages 34, 36, and 42, respectively.

The raw material liquids for Ba and Sr are mixed together by the liquidpump 22, which also works as a flow controller, and are sent to thevaporizer 24 where they are vaporized. The raw material gases for Ba andSr thus vaporized are mixed with Ar gas, i.e., a carrier gas, and theresultant mixed gases are supplied to the shower head 32 through the gaspassage 38.

The raw material liquid for Ti is sent by the liquid pump 44, which alsoworks as a flow controller, to the vaporizer 48 where it is vaporized.The raw material gas for Ti thus vaporized is mixed with Ar gas, i.e., acarrier gas, and the resultant mixed gases are supplied to the showerhead 32 through the gas passage 46.

A necessary amount of oxygen gas is supplied as an oxidizing gas, alongwith Ar gas, into the shower head 32. The atmosphere inside the processchamber 2 is vacuum-exhausted through the exhaust passage 14 and kept ata predetermined pressure by the pressure adjusting valve 12.

A detailed explanation will be given to the main part of the CVDapparatus with reference to FIGS. 2 and 3.

As described above, the CVD apparatus includes the process chamber 2formed of a cylinder of, e.g., aluminum. In the bottom 66 of the processchamber 2, there is a lead-line guide hole 70 at the center, and exhaustports 8 at the periphery. The exhaust ports 8 are connected to theexhaust passage 14 provided with the vacuum pump (not shown), trap 10,and pressure adjusting valve.

A load lock chamber 108 capable of being vacuum-exhausted is connectedto a side wall of the process chamber 2 by a gate valve 107. Asemiconductor wafer W is transferred into and out of the process chamber2 through the load lock chamber 108. The process chamber 2 and the loadlock chamber 108 are connected to a mechanism (not show) for supplyingN₂ gas for purging.

The worktable 4 arranged in the process chamber 2 is a cylindrical tablemade of a non-conductive material, such as alumina (Al₂ O₃) or AlN. Aleg portion 68 formed of a hollow cylinder is integratedly formed at thecenter of the bottom of the worktable 4 and extends downward. The lowerend of the leg portion 68 is arranged to surround the guide hole 70 inthe bottom 66 of the process chamber 2 and is airtightly connected andfixed to the bottom 66 by bolts 74 with a seal member 72, such as anO-ring, interposed therebetween. Consequently, the inside of the hollowleg portion 68 communicates with the outside of the process chamber 2,and is airtightly isolated from the inside of the process chamber 2.

A resistance heating body 76 is embedded in the top of the worktable 4,so that the wafer W mounted thereon can be heated to a predeterminedtemperature. Further, on the worktable 4, there is an electrostaticchuck 80 formed of a thin ceramic body in which an electrode 78 of,e.g., copper plate, is embedded. The wafer W is attracted and held onthe top of the worktable 4 by Coulomb's force generated by theelectrostatic chuck 80. A backside gas, such as He gas, is suppliedbetween the bottom of the wafer W and the surface of the electrostaticchuck 80, so that the heat conductivity to the wafer W is improved, andfilm deposition on the bottom of the wafer W is prevented. In place ofthe electrostatic chuck 80, a mechanical clamp may be employed.

The resistance heating body 76 is connected to a lead line 82 forsupplying electricity, which is insulated from the members around it.The lead line 82 is lead out to the outside of the process chamber 2through the cylindrical leg portion 68 and the guide hole 70, withoutbeing exposed to the inside of the process chamber 2, and is connectedto a power supply section 85 though a switch 84. The electrode 78 of theelectrostatic chuck 80 is connected to a lead line 86 for supplyingelectricity, which is insulated from the members around it. The leadline 86 is lead out to the outside of the process chamber 2 through thecylindrical leg portion 68 and the guide hole 70, without being exposedto the inside of the process chamber 2, and is connected to ahigh-voltage DC power supply 90 though a switch 88. In place of theresistance heating body 76, a heating lamp, such as a halogen lamp, maybe used for heating the wafer W.

A plurality of holes 92 are formed at positions on the periphery of theworktable 4 to penetrate the worktable 4, and lifter pins 94 arearranged in the holes 92 to be vertically movable. When the wafer W istransferred, the wafer w is moved in a vertical direction by anelevating mechanism (not shown) through the lifter pins 94. Generally,three lifter pins 94 are arranged to correspond to the periphery of thewafer W.

The shower head 32 arranged on the ceiling of the process chamber 2 hasa ceiling plate 96 airtightly attached to the top of the process chamber2 with a seal member 98, such as an O-ring, interposed therebetween. Theshower head 32 faces the worktable 4 to entirely cover the top surfaceof the worktable 4. A process field RF is defined between the showerhead 32 and the worktable 4. The shower head 32 is provided with anumber of spouting holes 102A and 102B in a shower region 100 on itsbottom surface.

The inside of the shower head 32 is divided into two spaces, i.e., aspace 32A for Ba and Sr, and a space 32B for Ti. The space 32A for Baand Sr has a port 104 connected to the gas passage 38 for introducingthe mixed raw material gases for Ba and Sr. The space 32B for Ti has aport 106 connected to the gas passage 46 for introducing the rawmaterial gas for Ti. Oxygen gas used as an oxidizing gas and Ar gas areintroduced into one or both of the spaces 32A and 32B. The spoutingholes 102A and 102B consist of two groups, i.e., a group of spoutingholes 102A connected to the space 32A for Ba and Sr, and a group ofspouting holes 102B connected to the space 32B for Ti. The process gasesspouted from the spouting holes 102A and 102B are mixed in the processfield PF (in a manner of so called post-mixing supply).

The spouting holes 102A for spouting a process gas, which generatesprecursors having lower activation energies and higher resistivities,i.e., the process gas for Ba and Sr in this embodiment, have a constantdiameter of, e.g., from about 1 to 2 mm, over the entirety of the showerregion 100, and are uniformly distributed. Consequently, the process gasfor Ba and Sr are uniformly spouted from the spouting holes 102A.

On the other hand, the spouting holes 102B for spouting a process gas,which generates precursors having higher activation energies and lowerresistivities, i.e., the process gas for Ti in this embodiment, areuniformly distributed, but have diameters gradually decreasing in radialdirections outward from the center of the shower region 100.Consequently, the process gas for Ti are spouted such that the gassupply amount or spouting amount is largest at the center and graduallydecreases toward the periphery.

In FIG. 3, the spouting holes 102A for the process gas for Ba and Sr,and the spouting holes 102B for the process gas for Ti are shown ascircles with hatching and circles without hatching, respectively. Thechange in the diameters of the spouting holes 102B for Ti varies inaccordance with gas types and process conditions for film deposition, sothat the composition ratio of metal elements in the process gasesmaintains a high planar uniformity on the wafer surface duringprocessing.

For example, where the wafer size is 8 inches, and the film depositiontemperature is about 500° C., the spouting holes 102A for Ba and Sr havea constant diameter L1 of from about 1 to 2 mm, while the spouting holes102B for Ti have diameters gradually decreasing in radial directions,such that a diameter L2 at the head center is from about 1.5 to 3 mm anda diameter L3 at the head periphery is from about 1 to 2 mm. Dependingon process conditions, such as the gas flow rates and process pressure,L1 and L3 are set at values falling in a range of from 1 to 2 mm, and L2is set at a value falling in a range of from 1.5 to 3 mm. The differencebetween L1 and L2 is set to be not more than 1.0 mm. Although thespouting holes 102A and 102B are arranged in a concentric format asshown in FIG. 3, they may be arranged in another format, such as across-strip format.

An explanation will be given to a CVD method performed in the CVDapparatus according to the first embodiment.

At first, a semiconductor wafer W is transferred from the load lockchamber 108 through the gate valve 106 into the process chamber 2, andmounted on the worktable 4. The wafer W is attracted and held on theworktable 4 by Coulomb's force generated by the electrostatic chuck 80,and is kept at a predetermined temperature of, e.g., from 400 to 600° C.by the resistance heating body 76. The process chamber 2 is kept at apredetermined pressure of, e.g., from 0.1 to 1 Torr, while the chamber 2is vacuum-exhausted, but is supplied with the process gases.

As described above, the process gas containing Ba and Sr is introducedinto the space 32A of the shower head 32 from the port 104, diffuses inthe space 32A, and is spouted from the spouting holes 102A into theprocess field PF. At this time, an oxidizing gas (oxygen gas) and Ar gasare simultaneously supplied into the space 32A, and are mixed with theprocess gas to be spouted into the process field PF in a shower fashion.Since the spouting holes 102A have a constant diameter and are almostuniformly distributed in the shower region 100, the process gascontaining mixed gases for Ba and SR is spouted at almost the samespouting rate per unit area over the entirety of the shower region 100.

On the other hand, the process gas containing Ti is introduced into theother space 32B of the shower head 32 from the port 106, diffuses in thespace 32B, and is spouted from the spouting holes 102B into the processfield PF. The process gas containing Ti is mixed with the process gascontaining Ba and Sr in the process filed PF. Since the spouting holes102B have diameters gradually decreasing in radial directions outwardfrom the head center, the spouting rate from spouting holes 102B at thehead center is larger than that from spouting holes 102B at the headperiphery.

As described above, the process gas containing Ti, which is lessreactive, is excessively supplied at the center and does not react somuch due to its low reactivity. Consequently, the process gas containingTi is transferred laterally and gradually spread onto the periphery ofthe wafer by means of diffusion or flow. The periphery is supplied withan insufficient amount of the process gas containing Ti due to the lowspouting rate from above, but is supplemented with the Ti rich processgas flowing from the center. As a result, the composition ratio of themetal elements in the atmospheric gas on the wafer surface becomesalmost the same between the center and the periphery. It follows thatthe film formed on the wafer surface can have an almost uniformcomposition ratio of the metal elements from the center to theperiphery. Further, the gas amount on the wafer surface is not sodifferent between the center and the periphery, and the film thicknesscan also be kept at a high planar uniformity.

FIG. 6 is a graph showing results of examining the composition of a filmformed by an example S1 of a method according to the first embodiment ofthe present invention, and the composition of a film formed by acomparative example S2 of a conventional method. In FIG. 6, thehorizontal and vertical axes denote the position on a wafer, and theratio (relative value) of Ti to (Ba+Sr), respectively. The conventionalmethod used here was a method in which the raw material gases for Ba,Sr, and Ti were all mixed in advance, and uniformly supplied from ashower head. The other conditions for film deposition in the example S1and the comparative example S2 were set in common with each other. Theconditions for the film deposition were as follows:

Wafer size: 8 inches,

Wafer temperature: 480° C.,

Film deposition pressure: 0.5 Torr,

Ba (thd)₂ :0.15 mol/L, 0.04 ml/min,

Sr (thd)₂ :0.15 mol/L, 0.04 ml/min,

Ti(O-ipr)₂ (thd)₂ :0.25 mol/L, 0.12 ml/min,

Carrier gas Ar: 200 scam,

O₂ :3 slm.

As shown in FIG. 6, the comparative example S2 shows a composition ratiowhich is gradually higher from the center of the wafer to the periphery,so the planar uniformity of the film composition is not so desirable. Incontrast the example S1 shows a composition ratio, which is almostconstant all over the wafer, so the planar uniformity of the filmcomposition is desirable.

This desirable planar uniformity of the film composition is obtained bythe above described method in which the process gas to generateprecursors having higher reactivities is uniformly supplied all over thewafer, while the process gas to generate precursors having lowerreactivities is supplied such that the gas supply amount is largest atthe center and gradually decreases toward the periphery. In this case,the periphery of the wafer is supplied with the process gas to generateprecursors having lower reactivities, which is the sum of the processgas from above and the process gas flowing from the center. As a result,a thin film of the composite material to be formed can have a highplanar uniformity in the film thickness and the film composition.

FIG. 4 is a view showing the flow of the process gases spouted from theshower head, and their metal element ratios.

For example, it is supposed that the process gases are supplied from thespouting holes 102A for Ba and Sr, and the spouting holes 102B for Ti,at ratios in the flow rate and composition of Ba:Sr:Ti=10:10:50 (1:1:5)at the center of the shower head 32. Where there is a gas flow loss of50% due to mass transfer in lateral directions, the process gases reachthe center of the wafer surface at ratios in the rate and composition ofBa:Sr:Ti=5:5:25 (1:1:5). If the process gases are consumed at a ratio ofBa:Sr:Ti=2:2:4 (1:1:2) in the film deposition reaction, the processgases are left non-reacted at ratios in the rate and composition ofBa:Sr:Ti=3:3:21 (1:1:7). The non-reacted process gases flow outward inparallel to the wafer surface.

On the other hand, it is supposed that the spouting holes 102B for Tihave an opening area at the periphery of the shower head 32, which istwo fifths smaller than that at the center. The process gases aresupplied from the spouting holes 102A for Ba and Sr, and the spoutingholes 102B for Ti, at ratios in the flow rate and composition ofBa:Sr:Ti=10:10:20 (1:1:2) at the periphery of the shower head 32. Theprocess gases spouted from the periphery of the shower head 32 receiveforces in lateral directions stronger than those to the gases at thecenter of the shower head 32. Where there is a gas flow loss of 80% dueto mass transfer in lateral directions, the process gases reach theperiphery of the wafer surface at ratios in the rate and composition ofBa:Sr:Ti=2:2:4 (1:1:2).

When the process gases flowing to the periphery of the wafer from aboveand the process gases flowing from the center join with each other, theyform ratios in the rate and composition of Ba:Sr:Ti=5:5:25 (1:1:5),which are the same as those at the center of the wafer. Where the centerand periphery of the wafer are provided with same film depositionconditions in other respects, the process gases are consumed at a ratioof Ba:Sr:Ti=2:2:4 (1:1:2) at the periphery of the wafer, as in thecenter of the wafer. Such a phenomenon is repeatedly caused from thecenter to the edge of the wafer surface, so both of the thickness andcomposition of the formed film can have a high planar uniformity, evenif the wafer has a large size. The wafer is not limited to a specificsize, but the present invention can be applied to a wafer having anysize, such as 6, 8, or 12 inches.

The number of spouting holes 102A and 102B is about several hundreds inpractice, depending on the size of a wafer to be processed, though onlysome of the spouting holes 102A and 102B are shown in the drawings.Accordingly, the diameters of the spouting holes 102B may graduallydecrease stepwise in radial directions such that a certain number ofspouting holes 102B belonging to each group have the same diameter,instead of the continuous decrease in diameter.

Although, in the first embodiment, the diameters of the spouting holes102B are changed, the density of the spouting holes 102B may be changed,as shown in FIG. 5, to cause the gas spouting rate to vary. In FIG. 5,spouting holes 102B for Ti have the same diameter all over a shower head32, but they are arranged to have a distribution density which graduallydecreases in radial directions from the center to the periphery of ashower region 100. With this arrangement, the gas spouting rate for Tialso gradually decreases from the center to the periphery.

Although, in the first embodiment, oxygen gas and Ar gas are mixed onlyinto the process gas for Ba and Sr, they may be mixed only into theprocess gas for Ti, or mixed into both of the process gases for Ba andSr, and for Ti. Instead, the shower head may be provided with anotherspace and spouting holes dedicated only to O₂ and Ar gas, so that the O₂and Ar gas is independently supplied into the process field. Thisarrangement allows flow control to be relatively easily performed, andthus suits a case where the supply amount of O₂ and Ar gas is small.

Further, in the first embodiment, since precursors of Ba and Sr havesimilar activation energies, the raw material gases for the two metalelements are mixed with each other in advance and are supplied into theprocess field. This arrangement allows the number of liquid pumps andvaporizers to be small, and the number of spaces in the shower head tobe reduced, thereby simplifying the apparatus. However, the shower headmay be provided with separate spaces and spouting holes dedicated to thegas for Ba and the gas for Sr, respectively, so that the two gases aresupplied into the process field independently of each other.

Although, in the first embodiment, a BST thin film is formed as acomposite material thin film, the present invention may be applied to aprocess of forming a PZT thin film, which contains Pb, Ar, Ti, and O.The PZT is an oxide of Pb (lead), Zr (zirconium), and Ti (titanium), andpresents a ferroelectric property when it has crystal of a perovskitestructure. Where a PZT thin film is formed by an MOCVD method accordingto the present invention, raw materials, such as Ti(iOPr)₄ =titaniumtetraisopropoxide, Zr(OtBt)₄ =tetra-t-butyl zirconium, and Pb(DPM)₂=bis(tetramethylheptanedionato)lead, and an oxidizing gas, such as NO₂,are used. Among precursors produced from the raw materials in filmforming reaction, precursors for Ti and Zr have higher activationenergies and lower reactivities, as compared to precursors for Pb.

The composition ratio of the PZT thin film is controlled to satisfyPb/(Ti+Zr)=1, and 1<Ti/Zr. The Ti/Zr ratio is hard to control so as tofall in a certain variation range, over the surface of a wafer, relativeto a reference ratio, such as 1.2. For example, with a conventionalmethod, in which the raw material gases for Pb, Zr, and Ti are mixedwith each other in advance of supply, it is difficult to allow the Ti/Zrratio in the atmosphere on the wafer to have a planar uniformity of ±2%or less.

In contrast, according to the first embodiment of the present invention,the three raw material gases are supplied from a shower head, separatelyfrom each other, i.e., a gas for Pb, a gas for Zr and Ti, and a gas forNO₂. With this arrangement, the Ti/Zr ratio can be set at a target valuewithin ±2% when supplied. Consequently, it is possible to form a PZTfilm having a high planar uniformity in the film composition, even wherethe wafer is large. A PZT film having a high planar uniformity in thefilm composition can have a uniform ferroelectric property, therebyproviding a reliable device on the wafer.

In order to improve planar uniformity in film composition, the firstembodiment may be also applied to a SBT thin film, i.e., a film of anoxide of Sr (strontium), Bi (bismuth), and Ta (tantalum), or a thin filmfurther containing Nb (niobium) in addition to them, which is aferroelectric body and attracts an attention as a promising material.

FIG. 9 is a diagram showing a CVD apparatus according to a secondembodiment of the present invention. FIG. 10 is a plan view showing thebottom surface of a shower head shown in FIG. 9. In this embodiment, aTa₂ O₅ film is formed, using tantalum alkoxide, i.e., Ta(OC₂ H₅)₅, as araw material gas, and O₂ gas as an oxidizing gas.

This CVD apparatus 114 includes a process chamber 116 formed of acylinder of, e.g., aluminum. In the bottom 118 of the process chamber116, there is a lead-line guide hole 120 at the center, and exhaustports 128 at the periphery. The exhaust ports 128 are connected to avacuum-exhaust system 126 including pumps for vacuum-exhausting theinside of the process chamber 116, such as a turbo-molecular pump 122and a dry pump 124. The exhaust ports 128 consist of a plurality of,e.g., four, exhaust ports 128 equidistantly arranged on one circle inthe bottom 118, and connected to the common vacuum-exhaust system 126.

A load lock chamber 196 capable of being vacuum-exhausted is connectedto a side wall of the process chamber 116 by a gate valve 198. Asemiconductor wafer W is transferred into and out of the process chamber116 through the load lock chamber 196. The process chamber 116 and theload lock chamber 196 are connected to a mechanism (not show) forsupplying N₂ gas for purging.

A worktable 130 arranged in the process chamber 116 is a cylindricaltable made of a non-conductive material, such as alumina. A leg portion132 formed of a hollow cylinder is integratedly formed at the center ofthe bottom of the worktable 130 and extends downward. The lower end ofthe leg portion 132 is arranged to surround the guide hole 120 in thebottom 118 of the process chamber 116 and is airtightly connected andfixed to the bottom 118 by bolts 136 with a seal member 134, such as anO-ring, interposed therebetween. Consequently, the inside of the hollowleg portion 132 communicates with the outside of the process chamber116, and is airtightly isolated from the inside of the process chamber116.

Resistance heating bodies 138 and 140 made of, e.g., carbon coated withSiC, are embedded in the worktable 130. The temperature of the worktable130 is controlled in two zones by the resistance heating bodies 138 and140, so that the wafer W mounted thereon can be heated to apredetermined temperature.

The resistance heating bodies 138 and 140 are connected to lead lines144 and 150, respectively, for supplying electricity, which areinsulated from the members around them. The lead lines 144 and 150 arelead out to the outside of the process chamber 116 through thecylindrical leg portion 132 and the guide hole 120, without beingexposed to the inside of the process chamber 116, and are connected topower supply sections 148 and 154 though switches 146 and 152,respectively. In place of the resistance heating bodies 138 and 140, aheating lamp, such as a halogen lamp, may be used for heating the waferW.

A plurality of holes 156 are formed at positions on the periphery of theworktable 130 to penetrate the worktable 130, and lifter pins 158 arearranged in the holes 156 to be vertically movable. When the wafer w istransferred, the wafer W is moved in a vertical direction by anelevating mechanism (not shown) through the lifter pins 158. Generally,three lifter pins 158 are arranged to correspond to the periphery of thewafer W.

A shower head 160 is arranged on the ceiling of the process chamber 116and has a ceiling plate 162 airtightly attached to the top of theprocess chamber 116 with a seal member 164, such as an O-ring,interposed therebetween. The shower head 160 has a bottom surface almostthe same as or larger than the top surface of the worktable 130 to faceand entirely cover it. A process field RF is defined between the showerhead 160 and the worktable 130. The shower head 160 is provided with anumber of spouting holes 168 and 170 in a shower region 166 on itsbottom surface.

The inside of the shower head 160 is divided into two spaces, i.e., aspace 172 for the raw material gas, and a space 174 for the oxidizinggas. The space 172 tfor the raw material gas has a port 176 connected toa raw material passage 178 extending from a bubbling apparatus (notshown) for introducing Ta(OC₂ H₅)₅ which has been vaporized by bubblingwith, e.g., He gas. The space 174 for Ti has a port 179 connected to thegas passage 180 for introducing O₂ gas. The spouting holes 168 and 170consist of two groups, i.e., a group of spouting holes 168 connected tothe space 172 for the raw material gas, and a group of spouting holes170 connected to the space 174 for the oxidizing gas. The raw materialgas and the oxidizing gas spouted from the spouting holes 168 and 102bare mixed in the process field PF (in a manner of so called post-mixingsupply). The raw material passage 178 is surrounded by, e.g., a tapeheater 182 to heat the gas flowing through the passage 178 at theliquefying temperature or more, so that the vaporized raw material gasis prevented from returning to a liquid state.

In conventional apparatuses, spouting holes 168 for a raw material gasand spouting holes 170 for an oxidizing gas are arranged to beessentially uniformly distributed in a zone having a surface area thesame as or slightly larger than that of a wafer. However, in thisapparatus according to the present invention, the spouting holes 168 forthe raw material gas are arranged in a zone smaller than that for thespouting holes 170 for the oxidizing gas. Specifically, as shown in FIG.10, the spouting holes 170 for the oxidizing gas are arranged to beessentially uniformly distributed in a circular zone 184W having asurface area the same as or slightly larger than that of the wafer W. Onthe other hand, the spouting holes 168 for the raw material gas arearranged to be essentially uniformly distributed in a circular zone 186,which is smaller than the zone 184W.

In order to prevent clogging, the spouting holes 168 generally have thesame large diameter d1 of from 0.8 to 10 mm. The spouting holes 170generally have the same small diameter d2 of from 0.2 to 0.3 mm. Thedistribution density per unit surface area of the groups of the spoutingholes 168 and 170 are set to be about one /cm² and about one to three/cm², respectively.

Although the arranging zone 186 of the spouting holes 168 for the rawmaterial gas is set to be smaller than that of conventional apparatuses,the raw material gas is supplied at a supply rate the same as that ofthe conventional apparatuses. Consequently, the spouting speed of theraw material gas from the spouting holes 168 is higher than that of theconventional apparatuses.

A cooling jacket 190 is formed in the side wall of the shower head 160,and supplied with a coolant, such as warm water of about 60° C. Withthis arrangement, the wall surface of the shower head 160 is kept at atemperature of, e.g., from 140 to 175° C., at which the raw material gasis prevented from being thermally dissociated while being prevented fromreturning to a liquid state. The distance between the shower head 160and the worktable 130 is set at about from 10 to 30 mm.

A cooling jacket 192 is also formed in the side wall of the processchamber 116, and supplied with a coolant, such as warm water of about60° C. With this arrangement, the wall surface of the process chamber116 is kept at a temperature of, e.g., from 140 to 175° C., at which theraw material gas is prevented from being thermally dissociated whilebeing prevented from returning to a liquid state.

An explanation will be given to a CVD method performed in the CVDapparatus according to the second embodiment.

At first, a semiconductor wafer W is transferred from the load lockchamber 196 through the gate valve 198 into the process chamber 116, andmounted on the worktable 130. The wafer W mounted on the worktable 130is kept at a predetermined process temperature by the resistance heatingbodies 138 and 140. The process chamber 116 is kept at a predeterminedprocess pressure, while the chamber 116 is vacuum-exhausted, but issupplied with the process gases.

A liquid organic compound, Ta(OC₂ H₅)₅, is vaporized by bubbling with Hegas and supplied as the raw material gas. The supply rate of this gas isas small as, e.g., several mg/min, depending on deposition rate. The rawmaterial gas flows once into the space 172 of the shower head 160, andthen is supplied into the process field PF from the spouting holes 168,which are arranged in the shower region 166 and have a larger diameter.

On the other hand, the oxygen gas or O₂ gas flows once into the space174 of the shower head 160, then is supplied into the process field PFfrom the spouting holes 170, which are arranged in the shower region 166and have a smaller diameter. The raw material gas and O₂ gas are mixedand react with each other in the process field PF, so that a tantalumoxide (Ta₂ O₅) film is deposited and formed. At this time, reactionbyproducts, such as CH₃ CHO, are produced.

For example, where the wafer W is an 8-inch wafer, the process pressureand process temperature of film deposition are set to be from about 0.2to 0.3 Torr, and from about 250 to 450° C., e.g., 400° C., respectively.The supply rates of the material gas, He gas, and O₂ gas are set to be15 mg/min, 300 sccm, and 1000 sccm, respectively.

According to the second embodiment, since the arranging zone 186 of thespouting holes 168 is smaller, the spouting speed of the raw materialgas from the spouting holes 168 is increased. Consequently, the rawmaterial gas reaches the wafer W without being thermally dissociated onthe way, so that the concentration of the raw material gas increases andthe concentration of reaction byproduct gases decreases on the wafersurface. Where the concentration of the byproducts, such as CH₃ CHO,decreases on the wafer surface, the possibility of the byproducts beingtaken into the film to be formed is lowered, thereby the insulationbreakdown voltage characteristic of the film is improved.

Where the arranging zone 186 of the spouting holes 168 and the zone 184Whaving almost the same surface area as the wafer W have diameters D1 andD2, respectively, the ratio D1/D2 is set to fall in a range of from 5/8to 7/8. Where the ratio is smaller than 5/8, the spouting speed of theraw material gas is so increased that the concentration of the rawmaterial gas at the center of the process field PF becomes too high. Itfollows that a metal oxide film comes to have a thickness larger at thewafer center, thereby deteriorating planar uniformity in the filmthickness on the wafer. In contrast, where the ratio is larger than 7/8to be closer to that of conventional apparatuses, reaction byproductsare not sufficiently prevented from being mixed into the film, therebydeteriorating the insulation breakdown voltage characteristic of themetal oxide film.

Note that, if the spouting speed of the raw material gas needs to besimply made high, it suffices that the supply pressure of the rawmaterial gas is high. However, in this case, the supply rate of the rawmaterial gas is greatly increased, and requires the supply rates of theother gases, such as He gas for bubbling and O₂ gas, to be greatlyincreased. Especially, where the wafer W is a 12-inch wafer, the gaseshave to be increased by a large amount; which does not work in practice.

FIG. 11 is a graph showing the result of evaluating the insulationbreakdown voltage of tantalum oxide films formed by an example S11 of amethod according to the second embodiment of the present invention, anda comparative example S12 of a conventional method. In FIG. 11, thehorizontal and vertical axes denote the effective film thickness ET, andthe insulation breakdown voltage BV, respectively. The conventionalmethod used here was a method in which both of the raw material gas andthe oxidizing gas are supplied from spouting holes uniformly distributedin a zone having the same surface area as a wafer, i.e., the zone 184Wshown in FIG. 10. The other conditions for film deposition in theexample S11 and the comparative example S12 were set in common with eachother. The conditions for the film deposition were as follows:

Wafer size: 6 inches,

Wafer temperature: 450° C.,

Film deposition pressure: 0.225 Torr,

Ta(OC₂ H₅)₅ : 15 mg/min

Carrier gas He: 250 sccm,

O₂ : 1000 sccm,

Spouting holes 168: within D1=4.5 inches (example S11): within D2=6inches (example S12)

Spouting holes 170: within D2=6 inches (examples S11 and S12).

As shown in FIG. 11, the example 11 provides insulation breakdownvoltages higher than those of the comparative example S12, and thuspresents better properties, without reference to the film thickness.Especially, even where the effective film thickness is as thin as about2.3 nm in the example 11, it provides a high insulation breakdownvoltage.

The concentration distribution of the raw material and a reactionbyproduct below a shower head was simulated according to the apparatusof the present invention, and a conventional apparatus. In the apparatusof the present invention, the spouting holes 168 were uniformly arrangedin the zone 186 with D1 of 6 inches, and the spouting holes 170 wereuniformly arranged in the zone 184W with D2 of 8 inches, relative to an8-inch wafer. In the conventional apparatus, each group of the spoutingholes 168 and the spouting holes 170 were uniformly arranged in the zone184W with D2 of 8 inches, relative to an 8-inch wafer. FIGS. 12A and 12Bare views each showing the concentration distribution of the rawmaterial gas (Ta(OC₂ H₅)₅ : Pet), according to the apparatus of thepresent invention, and the conventional apparatus, respectively. FIGS.13A and 13B are views each showing the concentration distribution of thereaction byproduct (CH₃ CHO), according to the apparatus of the presentinvention, and the conventional apparatus, respectively.

As shown in FIGS. 12A and 12B, the concentration of the raw material gasgradually decreases from the shower head 160 to the surface of the waferW. In the apparatus of the present invention, as shown in FIG. 12A, theconcentration of the raw material gas on the wafer is from about 1.8 to1.9×10⁻⁶ mol/m³. In the conventional apparatus, as shown in FIG. 12B,the concentration of the raw material gas on the wafer is from about 1.7to 1.8×10⁻⁶ mol/m³. Accordingly, in the apparatus of the presentinvention, more of the raw material gas reaches the wafer surfacewithout being dissociated on the way.

As shown in FIGS. 13A and 13B, the concentration of the reactionbyproduct gradually increases from the shower head 160 to the surface ofthe wafer W. In the apparatus of the present invention, as shown in FIG.13A, the concentration of the reaction byproduct on the wafer is fromabout 50 to 60×10⁻⁶ mol/m³. In the conventional apparatus, as shown inFIG. 13B, the concentration of the reaction byproduct on the wafer isfrom about 75 to 80×10⁻⁶ mol/m³. Accordingly, in the apparatus of thepresent invention, the reaction byproduct is more efficiently exhausted,due to a higher spouting speed of the raw material gas.

Other simulations were conducted, while changing the diameter D1 (seeFIG. 10) of the arranging zone of the spouting holes 168 for the rawmaterial gas. The results of the simulations will be explained withreference to FIGS. 14 to 16. In the simulations, an 8-inch wafer wasused, while the arranging zone of the spouting holes 170 for theoxidizing gas was set to have a diameter D2 of 8 inches. FIGS. 14 to 16include numerals shown along the lines A to L in which the numerals onthe left side denote the diameter D1 (inch) of the arranging zone of thespouting holes 168 and the numerals on the right side denote thediameter D2 (inch) of the arranging zone of the spouting holes 170. Forexample, the symbol (8-8) corresponds to the shower head of theconventional apparatus, and the symbol (6-8) corresponds to the showerhead of a typical apparatus of the present invention.

FIG. 14 is a graph showing the concentration distribution of the rawmaterial gas (Ta(OC₂ H₅)₅ : Pet). As shown in FIG. 14, the concentrationof the raw material gas decreases rapidly within a portion of from 80 to100 mm from the center of the wafer. The line D (8-8) representing theconventional apparatus shows an undesirably low concentration of the rawmaterial gas within a portion of 0 to 80 mm from the center of thewafer. On the other hand, the lines C, B, and A show desirable resultssuch that the concentrations of the raw material gas on the wafer inthose cases are higher than that of the line D, and the concentrationsincrease in the order of the lines C, B, and A, i.e., with a decrease inthe diameter D1.

FIG. 15 is a graph showing the concentration distribution of thereaction byproduct (CH₃ CHO). As shown in FIG. 15, the line E (8-8)representing the conventional apparatus generally shows an undesirableconcentration of the reaction byproduct as high as 7.0×10⁻⁷ mol/m³ ormore. On the other hand, the lines F to H according to the presentinvention show desirable concentrations of the reaction byproduct fromthe center to the periphery of the wafer as low as 6.0×10⁻⁷ mol/m³ orless. Further, in the order of the lines F to H, the concentrations ofthe reaction byproduct become lower, while the concentrations increasemore at the periphery of the wafer. Accordingly, it has been found that,with a decrease in the diameter D1 (see FIG. 10), the spouting speed ofthe raw material gas increases so that the reaction byproduct isexhausted more effectively at the center and periphery of the wafer.

FIG. 16 is a graph showing the deposition rate of tantalum oxide films.As shown in FIG. 16, the line L (8-8) representing the conventionalapparatus shows deposition rates of 1.10 nm/min and 0.95 nm/min, at thecenter and periphery of the wafer, respectively. The difference betweenthe deposition rates at the center and periphery of the wafer ispreferably small. On the other hand, the lines K to I according to thepresent invention show increasingly higher deposition rates in the orderof the lines K to I, because the concentration of the raw material gasis higher at the center of the wafer, as described above. In the case ofthe line I showing the highest deposition rate, the deposition rate atthe center of the wafer is 1.20 nm/min, while that at the periphery is0.95 nm/min.

Even in the case of the line I, however, the difference between thedeposition rates at the center and periphery of the wafer is 0.25nm/min. Such a difference does not deteriorate planar uniformity in thefilm thickness so much, but is thus permissible. It is not preferable,however, to enlarge the difference any more between the deposition ratesat the center and periphery of the wafer, thereby deteriorating planaruniformity in the film thickness. Judging from the above describedresults, the ratio D1/D2 between the diameters D1 and D2 shown in FIG.10 should be set to fall in a range of from 5/8 to 7/8, and preferablyat about 6/8.

Although the simulations were performed on 8-inch wafers, the ratio ofD1/D2 is not limited for the wafer size, but can be applied to wafers ofother sizes, such as 12 inches and 6 inches.

In the second embodiment, O₂ is used as the oxidizing gas, but O₃, NO₂,NO, or vaporized alcohol may be used instead. The second embodiment maybe applied to a case where a film of a metal nitride, such as TiN or WN,or a film of a metal, such as Pt, Ru, Ir, or Ti, is deposited. In thiscase, such the following complex compounds may be used as raw materialgases containing a metal. Those are tris(pentane-dionato)iridium=(C₅ H₇O₂)₃ Ir; tetrakis(diethyl-amido)titanium=[(CH₃ CH₂)₂ N]₄ Ti;bis(cyclopenta-dienyl)ruthenium=(C₅ H₅)₂ Ru,methylcyclopenta-dienyl(trimethyl)olatinum=(CH₃ C₃ H₄)(CH₃)₃ Pt.

Further, it is possible to apply the concept of the second embodiment toa process of forming a BST film, a PZT film, and the like, mentionedwith reference to the first embodiment.

For example, where a BST film is formed, a first process gas comprisingBa(thd)₂ and Sr(thd)₂, and a second process gas comprisingTi(O-iPr)(thd)₂ or Ti(thd)₂ are supplied from a narrower zone 186 (seeFIG. 10), while a third gas comprising an oxidizing gas, e.g., NO₂ issupplied from a wider zone 184W (see FIG. 10). In this case, the conceptof the first embodiment may be further applied to this process, suchthat the first and second gases are supplied from spouting holesdifferent from each other wherein the first gas is supplied fromspouting holes 102A (see FIG. 3) at a spouting rate substantiallyuniform all over, while the second process gas is supplied from spoutingholes 102B (see FIG. 3) at a spouting rate gradually decreasing inradial directions outward.

Where a PZT film is formed, a first process gas comprising Pb(DPM)₂, anda second process gas comprising Zr(OtBt)₄ and Ti(iOPr)₄ are suppliedfrom a narrower zone 186 (see FIG. 10), while a third gas comprising anoxidizing gas, e.g., NO₂ is supplied from a wider zone 184W (see FIG.10). In this case, the concept of the first embodiment may be furtherapplied to this process, such that the first and second gases aresupplied from spouting holes different from each other wherein the firstgas is supplied from spouting holes 102A (see FIG. 3) at a spouting ratesubstantially uniform all over, while the second process gas is suppliedfrom spouting holes 102B (see FIG. 3) at a spouting rate graduallydecreasing in radial directions outward.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

What is claimed is:
 1. A single-substrate-processing CVD apparatus forforming a thin film containing first and second metal elements on atarget substrate while supplying first and second process gasescontaining said first and second metal elements, respectively, aprecursor of said first metal element produced by dissociation of saidfirst process gas having a lower activation energy and a higherresistivity than a precursor of said second metal element produced bydissociation of said second process gas, said apparatus comprising:anairtight process chamber; a worktable arranged in said process chamberto mount said target substrate thereon; an exhaust system configured toexhaust said process chamber; and a supply system having a shower headconfigured to supply said first and second process gases into saidprocess chamber, wherein said shower head has a shower region facingsaid worktable, on which a group of first spouting holes for spoutingsaid first process gas and a group of second spouting holes for spoutingsaid second process gas are arranged, and wherein said supply system isdesigned such that said first process gas is supplied at a spouting ratesubstantially uniform over said shower region, and said second processgas is supplied at a spouting rate gradually decreasing in radialdirections outward from a center of said shower region.
 2. The apparatusaccording to claim 1, wherein said shower head comprises first andsecond spaces for storing said first and second process gases separatelyfrom each other, and said groups of said first and second spouting holesare connected to said first and second spaces, respectively.
 3. Theapparatus according to claim 1, wherein said group of said secondspouting holes are designed such that said second spouting holes havediameters gradually decreasing in radial directions outward from saidcenter of said shower region.
 4. The apparatus according to claim 1,wherein said group of said second spouting holes are designed such thatsaid second spouting holes have a distribution density graduallydecreasing in radial directions outward from said center of said showerregion.
 5. The apparatus according to claim 1, wherein said worktable isprovided with a heater for heating said target substrate.
 6. Asingle-substrate-processing CVD apparatus for forming a thin filmcontaining a metal element on a target substrate while supplying firstand second process gases containing said metal element and a non-metalelement for combining with said metal element, respectively, saidapparatus comprising:an airtight process chamber; a worktable arrangedin said process chamber to mount said target substrate thereon; anexhaust system configured to exhaust said process chamber; and a supplysystem having a shower head configured to supply said first and secondprocess gases into said process chamber, wherein said shower head has ashower region facing said worktable, on which a group of first spoutingholes for spouting said first process gas and a group of second spoutingholes for spouting said second process gas are arranged, and whereinsaid group of said first spouting holes are arranged to distribute in afirst zone having a surface area substantially smaller than a surfacearea of said target substrate is facing said shower region, and saidgroup of said second spouting holes are arranged to distribute in asecond zone concentric with said first zone and having a surface areasubstantially the same as or larger than said surface area of saidtarget substrate.
 7. The apparatus according to claim 6, wherein saidshower head comprises first and second spaces for storing said first andsecond process gases separately from each other, and said groups of saidfirst and second spouting holes are connected to said first and secondspaces, respectively.
 8. The apparatus according to claim 6, wherein aratio of said first zone to said surface area of said target substratefalls in a range of from 5/8 to 7/8.
 9. The apparatus according to claim6, wherein said worktable is provided with a heater for heating saidtarget substrate.